The present invention relates to a method for densifying or compacting fillers or other low bulk density materials. More particularly, the present invention relates to a method for incresing the bulk density of materials having a low bulk density by contacting the low bulk density material with a suitable liquid and thereafter evaporating the liquid so as to provide a more dense material.
Fumed silica is a well known reinforcing agent or filler commonly employed to improve the physical properties of silicone rubber. Basically, fumed silica comprises silicon dioxide particles in extremely finely divided form. The specific surface area typically ranges from about 100 to about 400 square meters per gram. Similarly, the density correspondingly is about 2 to 3 pounds per cubic foot.
Other materials having a low bulk density will be obvious to those skilled in the silicone art as well as many other arts such as organic rubber and the like and includes, for example, fumed alumina, carbon blacks and aerogels such as silica aerogel.
A major disadvantage of such low bulk density materials is that they are relatively expensive to ship and store. A compounder or formulator, for example, who requires large quantities of fillers must pay premium rates for shipping because shipping containers such as rail cars cannot contain large amounts on a weight basis. Moreover, once the compounder or formulator receives such filler he must pay for a suitable storage area such as a warehouse, silo or the like, which cost is, of course, ultimately passed on to the consumer or end-user.
Accordingly, it would be highly desirable to provide a means for increasing the bulk density of fillers and other low bulk density materials in order to reduce transportation and storage costs.
There are many mechanical means whereby the bulk density of a powdered material such as fumed silica is increased. One such device for densifying and granulating powdered materials is disclosed in Oldham et al., U.S. Pat. No. 3,114,930. Oldham et al. relies on the use of a vacuum to remove air from an aerated powdered material thereby reducing the density.
Carter, U.S. Pat. No. 3,664,385, compacts finely divided particulate matter by utilizing a rotating screw feeder whereas Leon et al. solve the problem of densification by employing a pair of opposed gas-permeable belts arranged to either side of a common axis so as to define a generally convergent densifying zone between their adjacent faces.
Other examples of mechanical densifying devices include Loffer, U.S. Pat. No. 3,632,247, Kongsgaarden, U.S. Pat. No. 4,126,423, and Kratel, U.S. Pat. No. 4,326,852.
While the bulk density of many materials can be increased by mechanical means, there nevertheless remains certain shortcomings and disadvantages. For example, in the case of fumed silica, if the density is increased by a mechanical apparatus beyond about 6 pounds per cubic foot, unacceptably high amounts of agglomerates and grit are formed. Moreover, it has also been found that such fumed silica no longer disperses as well in silicone polymer as the lower bulk density fumed silica.
It is also known that carbon black can be densified with water to produce a high bulk density material, however, such material has excessive grit therein and is not very effective as a reinforcing agent when compounded into silicone sealant and rubber formulations.
Fitzgerald et al. in U.S. patent application Ser. No. 539,587, filed Oct. 6, 1983, and assigned to the same assignee as the present invention provides an excellent nonmechanical solution to the foregoing problem by blending the fumed silica or other powdered material with a silicone polymer so as to provide free flowing powders having higher densities than can be obtained by mechanical means.
Lucas, U.S. Pat. No. 2,938,009, discloses that certain materials such as fumed silica can be treated with cyclic alkyl polysiloxanes to reduce structuring in curable organopolysiloxane compositions. In carrying out the treating process, it is desirable that the cyclic alkyl polysiloxane employed be sufficiently volatile so that at reasonable temperatures, for instance from 150.degree. to 350.degree. C., volatilization of the cyclic polysiloxane readily takes place so as to diffuse the polysiloxane through the silica particles. One method for obtaining treated filler is said to involve intimately contacting the filler with cyclic alkyl polysiloxanes in an amount ranging from 5 to 50% by weight of cyclic polysiloxane based on the weight of the filler being treated.
Lucas further teaches that treatment of fillers by his process leaves the bulk density of the treated filler essentially unchanged from the bulk density of the intitial untreated filler. Moreover, according to Lucas, this is in direct contrast with the results that are usually obtained by treating the filler with an organosilicon compound described in the prior art, for example, U.S. Pat. No. 2,665,264, where, in some instances the bulk density is less than half of the bulk density of the original untreated filler.
Those skilled in the art will appreciate that the process of Lucas is based on heating the filler/cyclopolysiloxane mixture in order to effect diffusion of the cyclopolysiloxane through the filler, that is, treatment of the filler depends upon the interaction of a gaseous reactant with a solid reactant.
The present applicant has surprisingly found that the interaction between a powdered material such as fumed silica and an organic or organosiloxane liquid, such as a cyclopolysiloxane as well as many other materials which preferably are non-polar or only slightly polar, and thereafter evaporating such liquid causes a substantial increase in the bulk density of the powdered material without causing unacceptably high amounts of agglomerates and grit.